CONTROL OF TOMATO MILDEW.

New Zealand Journal of Agriculture, Volume XXXVI, Issue 2, 20 February 1928, Page 99

CONTROL OF TOMATO MILDEW.

TRIALS IN AUCKLAND DISTRICT, SEASON 1927-28. Introduction. At the beginning of the present . season a request was received by the Horticulture Division from the Auckland Tomato-growers’ Association for a demonstration in the control of mildew on tomato-plants grown under glass in that district. It was stated that the trouble commenced about the month of October, and was so severe as to prevent the full crop being brought to maturity.. This demonstration was. carried out under the supervision of Mr. W. H. Rice, Orchard Instructor for the district, who furnishes the report which follows these introductory notes. In the Department’s bulletin “ Tomato-culture,” by W. H. Taylor, it is stated, “This disease.is caused by a humid atmosphere and high temperature ; the remedy is better ventilation.” A survey of the glasshouses for tomato crops in the Auckland District showed a strange deficiency in ventilators in these buildings, and obviously without them adequate ventilation cannot be given. The ventilator equipment for a house 14 ft. wide in a cold climate is totally inadequate in a warm climate, especially when the house is widened to 30 ft. or more, and planted close with a tomato crop which in the month of October is more than half full of dense vegetation. Those were found to be the usual conditions in this locality. In a few instances the ventilator equipment was less, and in one instance observed a large house had been built for this purpose without ventilators of any kind. Obviously under such conditions some growers are making unreasonable demands from science in these days, and the sooner the position is realized the better. Suitable ventilation depends on the number and size of properly placed ventilators. The requirements of this kind to be built into a house depend on the crop which is to be grown, the climate of the locality, and the size of the house. Cucumber-houses built 12 ft. to 14 ft. wide are often erected without any ventilators and work satisfactorily, although even they would be more easily operated with some available ventilation occasionally. The tomato crop, however, requires a dry, buoyant atmosphere especially, which makes the problem of ventilation quite a different one. The warm humid climate normally prevalent in Auckland is naturally an important factor in the problem also. If in the South Island it is found necessary to have continuous ventilation along both sides of the ridge of the tomato glasshouse, in the northern districts one would expect the same, only wider. In addition to this the houses generally in the northern districts are wider and often larger in other ways. This may very probably be an economy, but in extending the measurements it should be remembered that the proportion of ventilator area must be correspondingly increased. . Suitable manures and culture, as Mr. Rice shows, increase the resistance of the plants to mildew in some degree, but relief in the main

depends no doubt in reducing the humidity and temperature during the warmer periods in summer by means of a large increase in the ventilator areas available. —W. C. Hyde, Horticulturist, Horticulture Division. REPORT BY W. H. RICE. Throughout the Auckland District Cladosporium fulvum is much more prevalent on tomatoes under glass than in the colder parts of the Dominion. The disease makes an appearance on the plants towards the middle of October, and from then onward it is a race between the maturing of the larger part of the crop and the disease. Orthodox sprays have been tried in vain, and at present there seems no hope of control in that direction ; therefore endeavour is made to have the plants as forward as possible so that the greater part of the crop may be harvested. In this respect variable results have been obtained, and therefore experiments were undertaken this season and general observations specially made to determine, if possible, what factors in culture disposed the plants to susceptibility or resistance. The season has been a particularly bad one for Cladosporium. Continual wet weather and overcast conditions during the period of plant-growth (not very favourable for good ventilation) caused a tender growth of plants. As the fruits were swelling and ripening, continued open sunny weather conditions were general, with high day and low night temperatures, causing the disease to rapidly destroy the tender plants, even though well forward. Extra-tender plants harvested a much lesser' crop than might have been expected, and the fruit from, such plants was soft in texture owing to the absence of foliage. All the plants suffered more or less, but it has been outstanding that not only should the plants be well forward, but they should be well and hardily grown to afford maximum resistance to the disease. , VENTILATION. The majority of the glasshouses in the Auckland District are poorly ventilated. This causes more tender growth than advisable for tomatoes, and wherever possible it should be remedied. Observations show that the better the ventilation provided and the more freely it is used during the growing-period, the better the plants will resist Cladosporium. DUSTING. Comparisons were made with dry flowers of sulphur, Sulpho, and Cloud Form tomato-dust, as against undusted. Cladosporium was prevalent in each instance. Dusting with either of these materials cannot be considered as in any way beneficial. No variation in time of attack or severity could be noticed on any dusted section, which were all as bad as the undusted. The plants were fully defoliated by the disease by the time, the crop was ready for picking. DEFOLIATION AND PINCHING. The regular practice is to stop' plants beyond five bunches, all side shoots having been kept pinched out prior to this. About the time plants are stopped Cladosporium may be expected. To reduce infection

the lower leaves are pruned off as the lower fruits are picked. Observations show that the plants which fail to finish most of. their fruits are those on which all top-side laterals are suppressed, while those which are allowed to freely vegetate side shoots after the plants are stopped benefit greatly by this in growth, and more nearly perfect the whole of the crop. WHITE ISLAND MINERAL PRODUCT. Used as a soil-dressing prior to planting (300 lb. per acre) and with repeated surface applications after each watering or hoeing (100 lb. per acre) White Island “No. 1 Product” gave promising results as a soil insecticide. No plants were lost from the ravages of insects during establishment, as against a loss of- 12 per cent, in untreated land. Slugs, snails, cutworms, grass-grubs, and wood-lice were all controlled. The plants made good establishment, and throughout the whole period of growth were more robust than were those where this product was not used. The favourable growth was characterized by stouter stems, larger leaves, broader and denser foliage with an absence of crinkle at the edge, and firm and more uniform fruits. This favourable growth continued to resist Cladosporium some time after other plants were attacked. Though Cladosporium did finally develop, the crop was better all round and the season extended. In a section where the White Island product was not applied until some two weeks after planting, the plants did not show any better than the untreated until about 3 ft. high. From then onwards they began to show advantageously,, and finally made far better plants than the untreated. So marked was the better development that several growers decided to make late applications general. Some improvement was no doubt due to this, but it is very evident that early applications, at least two weeks prior to planting, give best results, though continued light dressings are favourable and advantageous. Many growers express their intention to continue the use of the product in future, both when the soil is prepared and in combination with fertilizers throughout the season. At the time of the. emergence of white-fly houses treated. were apparently free from infestation, indicating good control due to this product. During the period of Cladosporium development it was thought that the fumes liberated from the product were having a cleansing effect on the plants. In order to test this an extra liberal surface dressing was given to a whole house, but it was found that this method of using the product does not control Cladosporium. Its chief benefit appears to be in assisting to build up a robust plant. to resist the disease for a longer period, and as a soil-insect controllant. WATERING AND ATMOSPHERIC MOISTURE. Though Cladosporium is not usually prevalent until the warmer, dry weather, there is ample evidence to show that plants reared in a close humid atmosphere, causing tender growth, are more readily attacked and suffer more rapidly than hardier plants reared under dry conditions. Where plants are put out in dry or nearly dry land, and watered liberally periodically during growth, soft conditions are produced, and excess moisture is generally condensed on the glass. In order to test the effect of this moisture on Cladosporium a house was so arranged that while an adequate amount of root-moisture was

available the relative humidity of the atmosphere was as dry as possible., This was done by opening up furrows in the ground arid well watering several times prior to planting. ■ As the ground was levelled for planting, the surface soil was reasonably dry. The plants made very good establishment in growth,, requiring very little watering during the bad-weather period. As the weather • became more settled and warmer, more abundant water was supplied at regular intervals. Under this programme the house had a much drier atmosphere, and there was no condensation on the glass when the house was closeda desirable state of affairs, but one not readily attained when all water is applied during growth. The plants were well grown, of a hardy type, short-jointed, and resisted Cladosporium far better than more tender plants. FERTILIZERS. Complete fertilizers are necessary prior to or at planting, regardless of • what it is intended to apply later, . though at the earlier stage nitrogen should not be in the readily available forms. Young plants set out with dried blood alone were very stunted, -wooded, with copper-coloured foliage, and very backward six weeks after planting, showing no sign of flower-buds. With sulphate of potash alone the plants were -wooded, -jointed, more natural in colour, but not luxuriant, and showing bud development of flower. Superphosphate alone gave good growth and colour, with the plants showing open trusses of flower, but Cladosporium was present on most plants. Good, strong plants, very thrifty at the open-flower stage, resulted from complete fertilizers. K With regard to further dressings of fertilizers, generally super as a dominant has a pronounced effect in encouraging Cladosporium. Potash retards the trouble, but under its dominant influence plants do not come forward with sufficient vigour to be well established in the race against mildew. Nitrogen in dominance rushes the plants forward, and also has a retarding influence on the disease when applied, as the plants are at a susceptible period. SUMMARY. With present-known control measures Cladosporium may be expected in the district about mid-October. Endeavour should be made to have the plants forward and hardily grown prior to this. Dry atmospheric conditions should be maintained during winter, the majority of the water required being applied prior to planting, and good ventilation also maintained. The use of White Island product prior to planting, and soil-surface dressings during the season, appear to control soil insects and result in more robust plants capable of considerable resistance. Super in excess disposes the plants to Cladosporium. Complete fertilizers are advisable when the land is prepared for planting, together with nitrogen in a not immediately available form. Nitrogen should be freely used when the disease first appears. Top vegetation should be encouraged after stopping, so as to provide foliage to carry the plants through the attack.

Note. —The writer desires to record appreciation and thanks for the valuable assistance given in connection with these experiments by Mrs. A. Angus, Messrs. A. Currie, J. E. Fleet, G. Johnston, R. Lean, and White Island Products, Ltd.